1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus, and more particularly, to an image forming method and an image forming apparatus for recording a desired image by ejecting droplets onto a recording medium while moving a recording head (e.g. inkjet head) provided with a nozzle row which ejects ink droplets and the recording medium relatively with respect to each other.
2. Description of the Related Art
In order to record an image of high quality at high resolution with a serial type inkjet printer which records images onto a recording medium while a recording head is moved reciprocally, a recording head is known which has a composition where the ink ejection nozzles are increased in number only in respect of particular colors (for example, cyan and magenta), and these nozzles are disposed in symmetrical positions with respect to the center of the recording head (see FIG. 1 in Japanese Patent Application Publication No. 2005-313570).
By using a recording head having the nozzle arrangement disclosed in Japanese Patent Application Publication No. 2005-313570, when droplets of inks of a plurality of different types (colors) are ejected in a superimposed fashion in bidirectional printing, it is possible to ensure that the colors are superimposed on each other in substantially the same way (for example, printing cyan first and then yellow (i.e. printing in the order of “cyan→yellow”) and printing yellow first and then cyan (i.e. printing in the order of “yellow→cyan”) are carried out substantially the same number of times, respectively), and therefore the stability of secondary colors can be improved. Moreover, a merit is obtained in that when a high-quality mode is selected, it is possible to record at high resolution using all of the nozzle rows c1 to c4 (or nozzle rows m1 to m4), in respect of particular colors (for example, cyan and magenta).
However, in a recording head having the composition disclosed in the Japanese Patent Application Publication No. 2005-313570, when “normal recording mode” is selected, only the main nozzle rows c1 and c2 (or m1 and m2) are used. Therefore, if a recording head having a nozzle arrangement of this kind is installed at an oblique inclination with respect to the prescribed installation position (the standard installation position according to the design) and the head is then used in normal recording mode, the effective pitch between the nozzles will not be uniform, and there is a possibility that band-shaped non-uniformities may occur in the resulting image (recorded image).
FIG. 21 shows a schematic view of causes of the banding non-uniformities described above. Here, for the sake of convenience, only the cyan main nozzle rows c1 and c2, and the yellow nozzle rows y1 and y2 are depicted. The vertical direction in the drawing corresponds to the scanning direction of the recording head (main scanning direction), and the left/right direction in the drawing (the direction perpendicular to the main scanning direction) corresponds to the conveyance direction of the recording paper (sub-scanning direction).
If, as shown in FIG. 21, the nozzle rows of the same color are disposed in a spaced apart fashion on both ends of the recording head (in terms of the vertical direction) and a staggered nozzle arrangement is composed by means of these two nozzle rows, then there is a possibility that any inclination of the head (in other words, rotation within the ejection plane) will cause non-uniformity of the effective nozzle pitch (in other words, the pitch between the projected nozzles obtained by projection to an alignment in a direction perpendicular to the scanning direction of the recording head).
This variation in the nozzle pitch becomes more particularly marked, the greater the distance between the two nozzle rows of the same color (in other words, the greater the interval between the rows in the vertical direction in FIG. 21). The two rows in the vicinity of the center of the illustration in FIG. 21 (for example, the yellow nozzle rows y1 and y2) have a relatively short interval between the nozzle rows, and the variation in the effective nozzle pitch in the sub-scanning direction, Py1 and Py2, caused by any inclination (rotation) of the head is relatively small. In comparison with this, the two rows of the same color which are disposed on both ends in the upper and lower direction (for example, the cyan nozzle rows c1 and c2) have a large distance between the nozzle rows, and hence the variation in the nozzle pitch, Pc1 and Pc2, caused by any inclination (rotation) of the head becomes large. Consequently, if printing is carried out in this state, then band-shaped non-uniformities which are parallel to the main scanning direction can be perceived and can cause a great deterioration in image quality.